Java程序辅导

1School of Information Technologies / The University of Sydney        2011-S2
Software Development in Java
Week11 • Semester 2• 2015
Interfaces & Polymorphism
Practical Homework 3: in Lab, tonight
Assignment Submission: 6:00pm, Wed. 21 Oct
Assignment Demo: Tutorial, Wed. 21 Oct
School of Information Technologies / The University of Sydney        2011-S2
Review on Inheritance
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Inheritance
 Inheritance allows a software developer to derive a new class 
from an existing one
 The existing class is called the parent class, or superclass, or 
base class
 The derived class is called the child class or subclass
 As the name implies, the child inherits characteristics of the 
parent. That is, the child class inherits the methods and data 
defined by the parent class
School of Information Technologies / The University of Sydney        2011-S2
Inheritance
 We can write code in the superclass, and reuse it in the subclasses
without repeating it again
 Java keyword extends
 A class can only directly extend one parent
 It implicitly extends Object if nothing else is declared.
 The subclass inherits fields and methods of the parent
 Plus whatever is defined in the subclass itself
 Proper inheritance creates an is-a relationship, 
meaning the child is a more specific version of 
the parent
Person
Student
is
-a
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Abstract Class
 abstract method: no method body, e.g.:
public abstract void draw( );
 abstract class: a class that has an abstract method
 the subclasses provide implementation of an abstract 
method
 NO object can be created from an abstract class.
 Abstract classes enforce code reuse and software design.
School of Information Technologies / The University of Sydney        2011-S2
An Example: 2D shapes
We are going to define 2D shapes such 
as circles, rectangles and triangles
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Rectangle class (version 1)
public class Rectangle {
private double x; // x coordinate of bottom left hand corner
private double y; // y coordinate of bottom left hand corner
private double h; // height
private double w; // width
public double getX(){ ...
public double getY(){ ...
public double getWidth(){ ...
public double getHeight(){ ...
public Rectangle(double a, double b, double c, double d){
x=a; y=b; h=c; w=d;
}
public double area (){
return w*h;
}
public double perimeter (){
return 2*(w+h);
}
}
(x,y)
h
w
.
School of Information Technologies / The University of Sydney        2011-S2
Circle class (version 1)
public class Circle {
private double x; // (x,y) is the center of the circle
private double y;
private double r; // radius
public double getX(){ ...
public double getY(){ ...
public double getRadius(){ ...
public Circle(double a, double b, double c){
x=a; y=b; r=c;
}
public double area (){
return r*r*Math.PI;
}
public double perimeter (){
return 2*r*Math.PI;
}
}
(x,y)r
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Triangle class (version 1)
public class Triangle {
private double x; // x0
private double y; // y0; the points
private double x1; // (x0,y0),(x1,y1),(x2,y2)
private double y1; // are the
private double x2; // vertices of
private double y2; // the triangle
public double getX(){...
public double getY(){...
public double getX1(){...
public double getY1(){...
public double getX2(){...
public double getY2(){...
public Triangle(double a, double b, double c, double d, double e, double f){
x0=a;y0=b;x1=c;y1=d;x2=e;y2=f;
}
public double area (){
double a = Math.sqrt(((x0-x1)*(x0-x1) + (y0-y1)*(y0-y1)));
double b = Math.sqrt(((x0-x2)*(x0-x2) + (y0-y2)*(y0-y2)));
double c = Math.sqrt(((x2-x1)*(x2-x1) + (y2-y1)*(y2-y1)));
double s = 0.5*(a+b+c);
return Math.sqrt(s*(s-a)*(s-b)*(s-c));
}
public double perimeter (){ ...
(x0,y0)
(x1,y1)
(x2,y2)
School of Information Technologies / The University of Sydney        2011-S2
 Rectangles, circles, and 
triangles are all shapes.
 Rectangles, circles, and 
triangles all have
 A location (x,y)
 An area
 A perimeter
 Specifically, in addition:
 Rectangles have a 
height and a width
 Circles have a 
radius
 Triangles have two 
other vertices.
(x,y)
(x,y)
(x,y)
We can create a 
superclass to 
generalise
rectangles, circles, 
and triangles.
Use the concept of inheritance 
6School of Information Technologies / The University of Sydney        2011-S2
Use the concept of inheritance 
 We can define a superclass to generalize the shapes 
Practice on implementation by yourself!
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Java Interfaces
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Interfaces
 A java interface is not a class.
 An interface defines a type to generalize the common functions of similar 
classes
 An interface has a well defined API to list its functions by only specifying their 
names,  inputs and outputs and types, while without concrete implementation. 
 An interface includes a list of method signatures (name, parameters) and 
return type, but without method bodies
interface InterfaceName {
method signatures // but without any implementation
} 
interface Bicycle {
void changeGear(int newValue);
void speedUp(int increment);
void applyBrakes(int decrement);
} 
 There are many classes of bicycles.
 Internally, they have different 
methods of changing gears and 
applying brakes.
 But from the outside point of view, 
they have similar behaviour.
Syntax of interfaces:
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Implementing an Interface Type
 Use implements keyword to indicate that a class implements an 
interface type
 A class can implement more than one interface; 
 then the class must provide the method implementations for 
all the methods defined by all the interfaces that it 
implements
public class ClassName implements InterfaceName1, InterfaceName2, ... 
{ 
// methods (need to be declared as public) 
// fields 
}
8School of Information Technologies / The University of Sydney        2011-S2
interface Bicycle {
void changeGear(int newValue);
void speedUp(int increment);
void applyBrakes(int decrement);
} 
Java Interfaces
 Include a list of method signatures (name, parameters) and return type 
but without method body
 An interface can be implemented by a class. The class supplies method 
bodies for all the methods defined in the interface.
class GrannyBike implements Bicycle{
…
public void applyBrakes(int decrement) {
backPedal ();
}
. . … … 
class Racer implements Bicycle{
…
public void applyBrakes(int decrement) {
squeezeLever();
}
. . … … 
im
plem
entation
School of Information Technologies / The University of Sydney        2011-S2
interface Bicycle {
void changeGear(int newValue);
void speedUp(int increment);
void applyBrakes(int decrement);
} 
Java Interfaces
 No constructors
 Can be used to declare a reference variable
 But cannot be used to construct an object; and the object must 
constructed from a class that implements the interface
class GrannyBike implements Bicycle{
…
public void applyBrakes(int decrement){
backPedal ();
}
. . … … 
class Racer implements Bicycle{
…
public void applyBrakes(int decrement){
squeezeLever();
}
. . … … 
…
Bicycle b;
if (userAge>70) b = new GrannyBike();
else b = new Racer();
…
//client class
C
onstruct from
 
a class that 
im
plem
ented 
the interface 
9School of Information Technologies / The University of Sydney        2011-S2
2D Shapes Revisit
School of Information Technologies / The University of Sydney        2011-S2
Interfaces
 Rectangles, circles, and 
triangles are all shapes.
 Rectangles, circles, and 
triangles all have
 Getters for a given 
vertex (x,y)
 An area
 A perimeter
 Also:
 Rectangles have a 
height and a width
 Circles have a 
radius
 Triangles have two 
other vertices.
(x,y)
(x,y)
(x,y)
We can create an 
interface called 
Shape to generalise 
the functions of 
rectangles, circles, 
and triangles.
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Interfaces
 Note:interface Shape has
 some getters for instance variables, common to Rectangle, 
Circle, and Triangle
 some method signatures common to the classes Rectangle, 
Circle, and Triangle (methods area(), perimeter()), but no 
method bodies
 No constructors
public interface Shape {
double getX(); // getter: x coordinate of location
double getY(); // getter: y coordinate of location
double area ();
double perimeter ();
}
 Java interface gives a way of defining a generalization of the classes 
Rectangle, Circle, and Triangle
School of Information Technologies / The University of Sydney        2011-S2
Rectangle, Circle, and Triangle classes (version 2)
public class Circle implements Shape {
...
public double getX(){ //method body...
public double getY(){//method body...
...
public double area (){//method body...
public double perimeter (){//method body...
public class Triangle implements Shape {
...
public double getX(){//method body...
public double getY(){//method body...
...
public double area (){//method body ...
public double perimeter (){//method body...
public class Rectangle implements Shape
{
...
public double getX(){//method body...
public double getY(){//method body...
...
public double area (){//method body ...
public double perimeter (){//method body ...
public interface Shape {
double getX(); // x coordinate
double getY(); // y coordinate
double area ();
double perimeter ();
}
implements
(indicating these classes 
Must provide method 
implementations
for all the interface methods)
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School of Information Technologies / The University of Sydney        2011-S2
Circle class (version 2)
public class Circle implements Shape {
private double x; // the center of
private double y; // the circle is at (x,y)
...
public double getY(){ ...
public double getRadius(){....
.....
public double area (){
return r*r*Math.PI;
}
public double perimeter (){
return 2*r*Math.PI;
}
...
public interface Shape {
double getX(); // x coordinate
double getY(); // y coordinate
double area ();
double perimeter ();
}
implements
School of Information Technologies / The University of Sydney        2011-S2
Rectangle class (version 2)
public class Rectangle implements Shape {
private double x; // x coordinate
private double y; // y coordinate
...
public double getY(){ ...
public Rectangle(double a, double b, 
double c, double d){ ..
...
public double area (){
return w*h;
}
public double perimeter (){
return 2*(w+h);
}
...
public interface Shape {
double getX(); // x coordinate
double getY(); // y coordinate
double area ();
double perimeter ();
}
implements
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School of Information Technologies / The University of Sydney        2011-S2
Triangle class (version 2)
public class Triangle implements Shape {
private double x0; 
private double y0; 
...
public double getX(){ ...
public double getY(){ ...
...
public double getY2(){
...
public Triangle(double a, …
…
public double area (){
double a = Math.sqrt(((x0-x1)*(x0-x1) + (y0-y1)*(y0-y1)));
double b = Math.sqrt(((x0-x2)*(x0-x2) + (y0-y2)*(y0-y2)));
double c = Math.sqrt(((x2-x1)*(x2-x1) + (y2-y1)*(y2-y1)));
double s = 0.5*(a+b+c);
return Math.sqrt(s*(s-a)*(s-b)*(s-c));
}
public double perimeter (){
double a = Math.sqrt(((x0-x1)*(x0-x1) + (y0-y1)*(y0-y1)));
double b = Math.sqrt(((x0-x2)*(x0-x2) + (y0-y2)*(y0-y2)));
double c = Math.sqrt(((x2-x1)*(x2-x1) + (y2-y1)*(y2-y1)));
return a+b+c;
}
public interface Shape {
double getX(); // x coordinate
double getY(); // y coordinate
double area ();
double perimeter ();
}
implements
 When multiple classes 
implement the same 
interface, each class 
implements the 
methods of the interface 
in different ways
School of Information Technologies / The University of Sydney        2011-S2
Interfaces
Notes
 An interface defines the interface
of a set of classes, i.e. what to do, 
but it does not define how the 
classes work.
 Methods defined in an interface 
are automatically public.
 Corresponding methods in 
classes that implement the 
interface must be public
public interface Shape {
double getX(); // x coordinate
double getY(); // y coordinate
double area ();
double perimeter ();
}
public class Rectangle 
implements Shape {
...
private double h; // height
...
public double area (){
return w*h;
}
...
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School of Information Technologies / The University of Sydney        2011-S2
Interfaces
Notes
 Every Rectangle object is a specific 
Shape, and thus has all the functions 
of Shape
 Thereby Rectangles must have 
public methods
 getX(), with no parameters
 getY(), with no parameters
 area(), with no parameters
 perimeter(), with no parameters
 Rectangles may have instance 
variables and more public methods, 
but (since Rectangle implements 
Shape), they must have those
methods specified in the interface 
Shape.
public interface Shape {
double getX(); // x coordinate
double getY(); // y coordinate
double area ();
double perimeter ();
}
public class Rectangle implements 
Shape {
...
private double h; // height
...
public double area (){
return w*h;
}
...
School of Information Technologies / The University of Sydney        2011-S2
Circle class (version 3) with a toString method
public class Circle implements Shape {
private double x; // the center of
private double y; // the circle is at (x,y)
private double r; // radius
public double getX(){ ...
public double getY(){ ...
public double getRadius(){ ...
public Circle(double a, double b, double c){
x=a; y=b; r=c;
}
public double area (){ ...
public double perimeter (){ ...
public String toString(){
return "Circle center (" + x + "," + y + ") with radius " + r;
}
toString() : a method that 
represents an object by a string
Example: If an object c of type Circle has x=3.1, y=1.6, r=4.7, 
then c.toString() returns "Circle center (3.1,1.6) with radiu  r=4.7”
Similarly, toString methods
can be added to the 
Triangle and Rectangle
classes
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School of Information Technologies / The University of Sydney        2011-S2
Rectangle class (version 3) with a toString method
public class Rectangle implements Shape {
...
...
public String toString(){
return "Rectangle bottom L corner at (" + x + "," + y + 
") with height " + h + " and width " + w;
}
....
Example:
If an object rect of type Rectangle has x=5.4, y=7.3, h=6.1, and w=1.2, then 
rect.toString() returns 
“Rectangle bottom L corner at (5.4,7.3) with height 6.1 and width 1.2”
School of Information Technologies / The University of Sydney        2011-S2
A simple test class for interface Shape
public class ShapeTester {
public static void main(String[] args) {
Shape s;
s = new Rectangle(100, 100, 100, 200);
System.out.print(s);
System.out.println(" area=" + s.area() + " perimeter=" + s.perimeter());
System.out.println("---------");
s = new Circle(100, 100, 100);
System.out.print(s);
System.out.println(" area=" + s.area() + " perimeter=" + s.perimeter());
System.out.println("---------");
s = new Triangle(100, 100, 100, 200, 200, 200);
System.out.print(s);
System.out.println(" area=" + s.area() + " perimeter=" + s.perimeter());
}
}
Variable s has type Shape
s refers to an new object 
of type Rectangle
s refers to an new object 
of type Circle
s refers to an new 
object of type 
Triangle
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School of Information Technologies / The University of Sydney        2011-S2
A simple test class for interface Shape
public class ShapeTester {
public static void main(String[] args) {
Shape s;
s = new Rectangle(100, 100, 100, 200);
System.out.print(s);
System.out.println(" area=" + s.area() + " perimeter=" + s.perimeter());
System.out.println("---------");
s = new Circle(100, 100, 100);
System.out.print(s);
System.out.println(" area=" + s.area() + " perimeter=" + s.perimeter());
System.out.println("---------");
s = new Triangle(100, 100, 100, 200, 200, 200);
System.out.print(s);
System.out.println(" area=" + s.area() + " perimeter=" + s.perimeter());
}
}
Using the toString
method defined for 
each of the classes 
Rectangle, Circle, and 
Triangle
School of Information Technologies / The University of Sydney        2011-S2
public class ShapeTester {
public static void main(String[] args) {
Shape s;
s = new Rectangle(100, 100, 100, 200);
System.out.print(s);
System.out.println(" area=" + s.area() + " perimeter=" + s.perimeter());
System.out.println("---------");
s = new Circle(100, 100, 100);
System.out.print(s);
System.out.println(" area=" + s.area() + " perimeter=" + s.perimeter());
System.out.println("---------");
...
}
Rectangle bottom L corner at (100.0,100.0) with height 100.0 and width 200.0 
area=20000.0 perimeter=600.0
---------
Circle center (100.0,100.0) with radius 100.0 area=31415.926535897932 
perimeter=628.3185307179587
---------
...
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School of Information Technologies / The University of Sydney        2011-S2
Polymorphism via Interfaces
School of Information Technologies / The University of Sydney        2011-S2
Polymorphism via Interfaces
 An interface name can be used as the type of a reference variable.
 An interface reference variable can be used to refer to any 
object of any class that implements the interface.
 Polymorphism (many shapes): Behaviors can vary depending 
on the actual type (dynamic type) of an object, i.e., the actual 
type of the object determines the method implementation to be 
performed
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School of Information Technologies / The University of Sydney        2011-S2
Polymorphism via Interfaces
public class ShapeTester {
public static void main(String[] args) {
Shape s;
s = new Rectangle(100, 100, 100, 200);
System.out.print(s);
System.out.println(" area=" + s.area() + " perimeter=" + s.perimeter());
System.out.println("---------");
s = new Circle(100, 100, 100);
System.out.print(s);
System.out.println(" area=" + s.area() + " perimeter=" + s.perimeter());
System.out.println("---------");
s = new Triangle(100, 100, 100, 200, 200, 200);
System.out.print(s);
System.out.println(" area=" + s.area() + " perimeter=" + s.perimeter());
}
}
Variable s has type Shape
s refers to an new object 
of type Rectangle
s refers to an new 
object of type Circle
s refers to an 
new object of 
type Triangle
The same reference 
variable (s) can refer 
to different objects at 
different time. And 
hence, different 
methods (toString) 
are invoked 
correspondingly. 
 An interface reference variable can be used to refer to any object of 
any class that implements that interface. 
 The program varies according to the class that is plugged in– this 
phenomenon is called polymorphism
School of Information Technologies / The University of Sydney        2011-S2
Polymorphism via Interfaces
Note:  
 When we use an interface reference variable, we can invoke only the 
methods defined in the interface; otherwise, we will have a compiler 
error. 
public class Circle implements Shape {
private double x; // the center of
private double y; // the circle is at (x,y)
...
public double getY(){ ...
public double getRadius(){....
.....
public double area (){
return r*r*Math.PI;
}
public double perimeter (){
return 2*r*Math.PI;
}
...
public interface Shape {
double getX(); // x coordinate
double getY(); // y coordinate
double area ();
double perimeter ();
}
Shape sh;
sh = new Circle(100,100,100);
sh.getRadius(); //wrong!
Compiler error: the method 
getRadius() is undefined for the 
type Shape
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School of Information Technologies / The University of Sydney        2011-S2
 Going a bit further:
 A collection of shapes is a scene
School of Information Technologies / The University of Sydney        2011-S2
 Going a bit further:
 A collection of shapes is a scene
We know how to make a collection . . ... ... 
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School of Information Technologies / The University of Sydney        2011-S2
Scene class: instance fields, constructors, getters, and 
some private methods
import java.util.ArrayList; ...
public class Scene {
private ArrayList shapeList;
public Scene(){
shapeList = new ArrayList();
}
public Scene(String fileName){
shapeList = new ArrayList();
readScene(fileName);
}
public ArrayList getShapeList() {
return shapeList;
}
private void addShape (Shape s ) {
shapeList.add(s);
}
private void readScene(String fileName){ ...
....
....
getter
Two 
constructors
Instance field
Adds a shape to the scene
Reads the scene from a file
School of Information Technologies / The University of Sydney        2011-S2
Scene class: instance fields, constructors, getters, and 
some private methods
import java.util.ArrayList; ...
public class Scene {
private ArrayList shapeList;
public Scene(){
shapeList = new ArrayList();
}
public Scene(String fileName){
shapeList = new ArrayList();
readScene(fileName);
}
public ArrayList getShapeList() {
return shapeList;
}
private void addShape (Shape s ) {
shapeList.add(s);
}
private void readScene(String fileName){ ...
....
....
An interface name can be 
used as the type of a 
method parameter.
In such a case, any object 
of any class that 
implements the interface 
can be passed into the 
method. 
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School of Information Technologies / The University of Sydney        2011-S2
Scene class: 
private method to 
read a scene from 
a file
public class Scene {
...
private void readScene(String fileName){
try{
File file = new File(fileName);
Scanner fileReader = new Scanner(file);
while (fileReader.hasNextLine()){
String line = fileReader.nextLine();
Scanner lineReader = new Scanner(line);
String command = new String(lineReader.next());
if (command.equals("triangle")){
double x0 = lineReader.nextDouble();
...
addShape(new Triangle(x0,y0,x1,y1,x2,y2));
}
else if (command.equals("rectangle")){
...
addShape(new Rectangle(x,y,h,w));
}
else if (command.equals("circle")){
addShape(new Circle(x,y,r));
}
else System.out.println("Unknown shape: skipping");
}
} ...
An interface name can be 
used as the type of a method 
parameter. 
In such a case, any object of 
any class that implements 
the interface can be passed 
into the method. 
School of Information Technologies / The University of Sydney        2011-S2
Scene class: how to draw a scene
import javax.swing.*;
public class Scene { ...
...
public void draw(){
for (Shape s: shapeList) {
s.draw();
}
}
...
import javax.swing
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School of Information Technologies / The University of Sydney        2011-S2
Testing the Scene class
public class SceneTester {
public static void main(String[] args) {
Scene s = new Scene("C:/temp/temp.txt");
s.draw();
for (Shape t: s.getShapeList()){
System.out.println(t);
}
}
}
Triangle with vertices at (0.0,0.0)-(100.0,100.0)-(100.0,200.0)
Triangle with vertices at (0.0,100.0)-(200.0,200.0)-(100.0,400.0)
Circle center (400.0,400.0) with radius 200.0
Circle center (100.0,300.0) with radius 560.0
Rectangle bottom L corner at (100.0,200.0) with height 500.0 and width 600.0
Rectangle bottom L corner at (200.0,300.0) with height 400.0 and width 500.0
triangle 0 0 100 100 100 200
triangle 0 100 200 200 100 400
circle 400 400 200
circle 100 300 560
rectangle 100 200 500 600
rectangle 200 300 400 500
Input file C:/temp/temp.txt
O
ut
pu
t
Output
School of Information Technologies / The University of Sydney        2011-S2
Abstract Class versus Interface
 They are both used to enforce code reuse and class design.
 Abstract class can have concrete methods but interface does not.
 A class  
 extends only one abstract class
 implements many interfaces.
 A class declared as abstract does not have to contain abstract 
methods -- simply declaring the class as abstract.
 An interface type does not have instance fields, but can have 
static fields. 
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School of Information Technologies / The University of Sydney        2011-S2
Interface Hierarchies
 Inheritance can be applied to 
interfaces as well as classes
 That is, one interface can be 
derived from another interface (by 
extends)
 The child interface inherits all 
abstract methods of the parent
 A class implementing the child 
interface must define all methods 
from both the ancestor(s) and 
child interfaces
extends
implements